﻿/************************************************************************
 * SMLib - State Model Library
 * Copyright (C) 2012-2013 Ali Khalili (khalili_DOT_ir_@SIGN_gmail_DOT_com)
 * **********************************************************************
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License (LGPL) published 
 * by the Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version. This program is distributed without any
 * warranty. See the GNU Lesser General Public License for more details. 
 * 
 * This file is an adoptation of C code: (c) Bob Jenkins
 * http://burtleburtle.net/bob/c/lookup3.c
 * [see below for the licence]
 * ***********************************************************************
 * File         :     SMLib\Utils\Hashing\HashAlgos_Lookup3.cs
 * Description  :     Lookup3c hash algorithm
 * Change log   :     -Jan 2012:  First version
 * To Do        :     
 ************************************************************************/

/*
Copyright 2008-2010 the T2 Project ant the Others.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
/**
 * Produces 32-bit hash for hash table lookup.
 * 
 * <pre>
 * lookup3.c, by Bob Jenkins, May 2006, Public Domain.
 * You can use this free for any purpose.  It's in the public domain.
 * It has no warranty.
 * </pre>
 * 
 * @see <a href="http://burtleburtle.net/bob/c/lookup3.c">lookup3.c</a>
 * @see <a href="http://www.ddj.com/184410284">Hash Functions (and how this
 *      function compares to others such as CRC, MD?, etc</a>
 * @see <a href="http://burtleburtle.net/bob/hash/doobs.html">Has update on the
 *      Dr. Dobbs Article</a>
 */

using System;

namespace SMLib.Utils.Hashing
{
    /// <summary>
    /// Produces 32-bit hash for hash table lookup using Lookup3 algorithm
    /// <remarks>
    /// This code is adopted from the following Java code:
    /// http://www.java2s.com/Code/Java/Development-Class/Produces32bithashforhashtablelookupJenkinsHashFunction.htm
    /// </remarks>
    /// </summary>
    public static class HashAlgos_Lookup3
    {
        #region Lookup3 Algorithm

        private static long INT_MASK = 0x00000000ffffffffL;

        private static long BYTE_MASK = 0x00000000000000ffL;

        private static long rot(long val /*val*/, int pos /* pos*/)
        {
            //??return ((Integer.rotateLeft((int) (val & INT_MASK), pos)) & INT_MASK);
            return ((val & INT_MASK) << pos) & INT_MASK;
            //return (((val) << (pos)) | ((val) >> (32 - (pos))));
        }

        /**
         * taken from hashlittle() -- hash a variable-length key into a 32-bit value
         * 
         * @param key
         *            the key (the unaligned variable-length array of bytes)
         * @param nbytes
         *            number of bytes to include in hash
         * @param initval
         *            can be any integer value
         * @return a 32-bit value. Every bit of the key affects every bit of the
         *         return value. Two keys differing by one or two bits will have
         *         totally different hash values.
         * 
         *         <p>
         *         The best hash table sizes are powers of 2. There is no need to do
         *         mod a prime (mod is sooo slow!). If you need less than 32 bits,
         *         use a bitmask. For example, if you need only 10 bits, do
         *         <generatedCode>h = (h & hashmask(10));</generatedCode> In which case, the hash
         *         table should have hashsize(10) elements.
         * 
         *         <p>
         *         If you are hashing n strings byte[][] k, do it like this: for
         *         (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
         * 
         *         <p>
         *         By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use
         *         this generatedCode any way you wish, private, educational, or commercial.
         *         It's free.
         * 
         *         <p>
         *         Use for hash table lookup, or anything where one collision in
         *         2^^32 is acceptable. Do NOT use for cryptographic purposes.
         */
        //@SuppressWarnings("fallthrough")
        public static int hash(byte[] key, int nbytes, int initval)
        {
            int length = nbytes;
            long a, b, c; // We use longs because we don't have unsigned ints
            a = b = c = (0x00000000deadbeefL + length + initval) & INT_MASK;
            int offset = 0;
            for (; length > 12; offset += 12, length -= 12)
            {
                a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
                a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK))
                    & INT_MASK;
                a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK))
                    & INT_MASK;
                a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK))
                    & INT_MASK;
                b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
                b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK))
                    & INT_MASK;
                b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK))
                    & INT_MASK;
                b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK))
                    & INT_MASK;
                c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
                c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK))
                    & INT_MASK;
                c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK))
                    & INT_MASK;
                c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK))
                    & INT_MASK;

                /*
                 * mix -- mix 3 32-bit values reversibly. This is reversible, so any
                 * information in (a,b,c) before mix() is still in (a,b,c) after
                 * mix().
                 * 
                 * If four pairs of (a,b,c) inputs are run through mix(), or through
                 * mix() in reverse, there are at least 32 bits of the output that
                 * are sometimes the same for one pair and different for another
                 * pair.
                 * 
                 * This was tested for: - pairs that differed by one bit, by two
                 * bits, in any combination of top bits of (a,b,c), or in any
                 * combination of bottom bits of (a,b,c). - "differ" is defined as
                 * +, -, ^, or ~^. For + and -, I transformed the output delta to a
                 * Gray generatedCode (a^(a>>1)) so a string of 1's (as is commonly produced
                 * by subtraction) look like a single 1-bit difference. - the base
                 * values were pseudorandom, all zero but one bit set, or all zero
                 * plus a counter that starts at zero.
                 * 
                 * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
                 * satisfy this are 4 6 8 16 19 4 9 15 3 18 27 15 14 9 3 7 17 3
                 * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for
                 * "differ" defined as + with a one-bit base and a two-bit delta. I
                 * used http://burtleburtle.net/bob/hash/avalanche.html to choose
                 * the operations, constants, and arrangements of the variables.
                 * 
                 * This does not achieve avalanche. There are input bits of (a,b,c)
                 * that fail to affect some output bits of (a,b,c), especially of a.
                 * The most thoroughly mixed value is c, but it doesn't really even
                 * achieve avalanche in c.
                 * 
                 * This allows some parallelism. Read-after-writes are good at
                 * doubling the number of bits affected, so the goal of mixing pulls
                 * in the opposite direction as the goal of parallelism. I did what
                 * I could. Rotates seem to cost as much as shifts on every machine
                 * I could lay my hands on, and rotates are much kinder to the top
                 * and bottom bits, so I used rotates.
                 * 
                 * #define mix(a,b,c) \ { \ a -= c; a ^= rot(c, 4); c += b; \ b -=
                 * a; b ^= rot(a, 6); a += c; \ c -= b; c ^= rot(b, 8); b += a; \ a
                 * -= c; a ^= rot(c,16); c += b; \ b -= a; b ^= rot(a,19); a += c; \
                 * c -= b; c ^= rot(b, 4); b += a; \ }
                 * 
                 * mix(a,b,c);
                 */
                a = (a - c) & INT_MASK;
                a ^= rot(c, 4);
                c = (c + b) & INT_MASK;
                b = (b - a) & INT_MASK;
                b ^= rot(a, 6);
                a = (a + c) & INT_MASK;
                c = (c - b) & INT_MASK;
                c ^= rot(b, 8);
                b = (b + a) & INT_MASK;
                a = (a - c) & INT_MASK;
                a ^= rot(c, 16);
                c = (c + b) & INT_MASK;
                b = (b - a) & INT_MASK;
                b ^= rot(a, 19);
                a = (a + c) & INT_MASK;
                c = (c - b) & INT_MASK;
                c ^= rot(b, 4);
                b = (b + a) & INT_MASK;
            }

            // -------------------------------- last block: affect all 32 bits of
            // (c)
            switch (length)
            { // all the case statements fall through
                case 12:
                    c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 11:
                    c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 10:
                    c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 9:
                    c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
                    break;//in C#
                case 8:
                    b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 7:
                    b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 6:
                    b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 5:
                    b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
                    break;//in C#

                case 4:
                    a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 3:
                    a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 2:
                    a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK))
                        & INT_MASK;
                    break;//in C#
                case 1:
                    a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
                    break;//:O it was here
                case 0:
                    return (int)(c & INT_MASK);
            }
            /*
             * final -- final mixing of 3 32-bit values (a,b,c) into c
             * 
             * Pairs of (a,b,c) values differing in only a few bits will usually
             * produce values of c that look totally different. This was tested for
             * - pairs that differed by one bit, by two bits, in any combination of
             * top bits of (a,b,c), or in any combination of bottom bits of (a,b,c).
             * 
             * - "differ" is defined as +, -, ^, or ~^. For + and -, I transformed
             * the output delta to a Gray generatedCode (a^(a>>1)) so a string of 1's (as is
             * commonly produced by subtraction) look like a single 1-bit
             * difference.
             * 
             * - the base values were pseudorandom, all zero but one bit set, or all
             * zero plus a counter that starts at zero.
             * 
             * These constants passed: 14 11 25 16 4 14 24 12 14 25 16 4 14 24 and
             * these came close: 4 8 15 26 3 22 24 10 8 15 26 3 22 24 11 8 15 26 3
             * 22 24
             * 
             * #define final(a,b,c) \ { c ^= b; c -= rot(b,14); \ a ^= c; a -=
             * rot(c,11); \ b ^= a; b -= rot(a,25); \ c ^= b; c -= rot(b,16); \ a ^=
             * c; a -= rot(c,4); \ b ^= a; b -= rot(a,14); \ c ^= b; c -= rot(b,24);
             * \ }
             */
            c ^= b;
            c = (c - rot(b, 14)) & INT_MASK;
            a ^= c;
            a = (a - rot(c, 11)) & INT_MASK;
            b ^= a;
            b = (b - rot(a, 25)) & INT_MASK;
            c ^= b;
            c = (c - rot(b, 16)) & INT_MASK;
            a ^= c;
            a = (a - rot(c, 4)) & INT_MASK;
            b ^= a;
            b = (b - rot(a, 14)) & INT_MASK;
            c ^= b;
            c = (c - rot(b, 24)) & INT_MASK;

            return (int)(c & INT_MASK);
        }

        #endregion
    }


}
